U.S. patent application number 11/094168 was filed with the patent office on 2005-10-06 for optical pulse evaluation device and in-service optical pulse evaluation device.
This patent application is currently assigned to santec corporation. Invention is credited to Hirota, Yoichi, Ozeki, Yasuyuki, Takushima, Yuichi, Uehara, Noboru.
Application Number | 20050219543 11/094168 |
Document ID | / |
Family ID | 35053921 |
Filed Date | 2005-10-06 |
United States Patent
Application |
20050219543 |
Kind Code |
A1 |
Uehara, Noboru ; et
al. |
October 6, 2005 |
Optical pulse evaluation device and in-service optical pulse
evaluation device
Abstract
To obtain an optical pulse evaluation device and an in-service
optical pulse evaluation device, which are capable of
characteristics evaluation of an optical pulse itself or a sample
launched therein, in a relatively high bit-rate region. Optical
pulse 42 output repeatedly from an optical pulse light source 43 at
a frequency fREP passes through sample 93 n sample stage 91 to be
scanned by tunable wavelength optical band pass filter 47. A
detection result by photodiode 51 is input in phase detection
circuit 45 accompanying with the reference frequency fREP and the
result is operated by operation unit 58C of personal computer 52 to
know a spectral phase and a spectral intensity of the optical pulse
passed through sample 93. Correcting this by applying the operation
result in the state, where sample 93 has been removed enables to
evaluate characteristics such as deterioration of optical pulse 42
by using sample 93. It is also possible to evaluate a waveform of
optical pulse 42 as a light source. The present invention can
evaluate the optical pulse in in-service.
Inventors: |
Uehara, Noboru; (Aichi,
JP) ; Takushima, Yuichi; (Tokyo, JP) ; Ozeki,
Yasuyuki; (Tokyo, JP) ; Hirota, Yoichi;
(Tokyo, JP) |
Correspondence
Address: |
RADER FISHMAN & GRAUER PLLC
LION BUILDING
1233 20TH STREET N.W., SUITE 501
WASHINGTON
DC
20036
US
|
Assignee: |
santec corporation
|
Family ID: |
35053921 |
Appl. No.: |
11/094168 |
Filed: |
March 31, 2005 |
Current U.S.
Class: |
356/450 |
Current CPC
Class: |
G01J 11/00 20130101 |
Class at
Publication: |
356/450 |
International
Class: |
G01B 009/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 2004 |
JP |
2004-103835 |
Mar 3, 2005 |
JP |
2005-059559 |
Claims
What is claimed is:
1. An optical pulse evaluation device comprising: an optical pulse
outputting means for outputting an optical pulse to be evaluated;
an optical frequency component extracting means for extracting a
specific optical frequency component of the optical pulse output
from this optical pulse outputting means; a frequency component
intensity measurement means for measuring an intensity of the
specific optical frequency component of the optical pulse extracted
by this optical frequency component extracting means; and a phase
intensity operating means for operating the spectral phase and the
spectral intensity of the optical pulse output from said optical
pulse output means on the basis of a measurement result by this
frequency component intensity measurement means.
2. The optical pulse evaluation device comprising: the optical
pulse outputting means for outputting the optical pulse to be
evaluated; an optical dividing means for dividing the optical pulse
output from this optical pulse outputting means; the optical
frequency component extracting means for inputting the one of
optical pulses divided by this optical dividing means to extract
the specific optical frequency component; the frequency component
intensity measurement means for measuring intensity of the specific
optical frequency component of the optical pulse extracted by this
optical frequency component extracting means; a whole optical
intensity measurement means for measuring the intensity of a whole
of the other optical pulse divided by said optical dividing means;
and the phase intensity operating means for operating the spectral
phase and the spectral intensity of the optical pulse output from
said optical pulse output means on the basis of the measurement
result of said frequency component intensity measurement means and
the whole optical intensity measurement means.
3. The optical pulse evaluation device according to claim 2,
wherein said optical pulse outputting means comprises a frequency
defining means for defining a repetition frequency of a pulse train
and a pulse light source for converting a continuum by the
repetition frequency defined by this frequency defining means to
output as the pulse train.
4. The optical pulse evaluation device according to claim 2, having
an error eliminating means for eliminating an error, which is
produced by the dispersion and transmittance characteristic of said
optical frequency component extracting means, by arithmetic
operation.
5. The optical pulse evaluation device according to any one of
claim 2, wherein: said optical frequency component extracting means
is a band pass filter and has a band scanning means for changing
sequentially the specific optical frequency component, which is
extracted by this band pass filter, across at least a partial band
of the whole frequency bands of said optical pulse; and said phase
intensity operating means operates a waveform of said optical pulse
on the basis of the spectral phase and the spectral intensity in
individual optical frequency components obtained from a scanning
result by said band scanning means.
6. The optical pulse evaluation device according to claim 5, having
a secondary operating means for operating at least any one of a
pulse width, a chirping, a dispersion, and a phase shift of said
optical pulse by using the spectral phase and the spectral
intensity obtained by said phase intensity operating means.
7. The optical pulse evaluation device according to claim 6,
further having a measurement sample locating means for locating a
measurement sample mountably and unmountably in a light path
linking said optical pulse outputting means to said frequency
component intensity measurement means; and a sample operating means
for operating at least any one of (chirping, a dispersion, and a
phase shift, which is cased by said measurement sample, of a
reference optical pulse) and (a transmittance characteristic,
dispersion, and a nonlinear effect of said measurement sample) by
using an operation result of aid phase intensity operating means in
a state where this measurement sample is located and the state
where not located in said light path.
8. The optical pulse evaluation device according to claim 2,
wherein said optical pulse outputting means is the means for
outputting the optical pulse, of which light intensity or phase is
modulated by a digital data signal.
9. The optical pulse evaluation device according to claim 8,
wherein said digital data signal contains a pseudo random
signal.
10. The optical pulse evaluation device according to claim 8,
further having: a clock extraction means for extracting a clock
signal synchronized with said digital data signal on the basis of
the measurement result by said frequency component intensity
measurement means; a phase change detection means for detecting a
phase change of said clock signal extracted by this clock
extraction means; a spectrum phase operating means for calculating
a spectral phase from a phase change detected by this phase change
detection means.
11. The optical pulse evaluation device according to claim 2,
wherein said optical pulse outputting means is a means for
outputting an optical pulse, of which light intensity or phase is
modulated by the digital data signal, said optical frequency
component extracting means is a band pass filer, and further having
a band scanning means for changing sequentially the specific
optical frequency component, which is extracted by this band pass
filter, across at least a partial band of a whole frequency bands
of said optical pulse; and said spectral phase operating means for
calculating a spectral phase of an optical pulse train by
extracting a repetition frequency component of band scanning from a
phase change of a clock signal yielded by a scanning result by this
band scanning means.
12. The optical pulse evaluation device according to claim 11,
having a chirping operating means for calculating a chirping of
said optical pulse by using the spectrum phase operated by said
spectrum phase operating means.
13. An in-service optical pulse evaluation device comprises: an
optical modulation unit for modulating a light ray, which is
emitted from a light source, by using a digital data signal
synchronized with a predetermined clock; an optical divider for
dividing an optical pulse train, which is obtained by modulation of
this optical modulation unit, in a first and a second paths by
using a transmission line; a band pass filter which is located in
the first path for inputting the optical pulse train obtained by
modulation of said optical modulation unit; a sweeping unit for
sweeping a center frequency of this band pass filter; a first
photoelectric conversion element for receiving the light with the
wavelength component passed through said band pass filter; a first
clock extraction module for inputting the electric signal converted
by this first photoelectric conversion element to extract a clock
signal synchronized with the digital data transmitted; a second
photoelectric conversion element for receiving said optical pulse
train transmitted through said second path; a second clock
extraction module for inputting the electric signal converted by
this second photoelectric conversion element to extract the clock
signal synchronized with the digital data transmitted; a phase
detection means for detecting the phase of the clock signal output
by the first and the second clock extraction modules to calculate
time delayed by said band pass filter; and an operation means for
measuring the measurement result by the phase detection means
during sweeping the center frequency of the band pass filter by
said sweeping unit to operate chirping of the optical pulse or a
dispersion of said transmission line.
14. The in-service optical pulse evaluation device comprises: the
optical modulation unit for modulating the light ray, which is
emitted from the light source, by using the digital data signal
synchronized with the predetermined clock; a signal sending unit
for sending the optical pulse train, which is obtained by
modulation of the optical modulation unit, to the transmission
line; the band pass filter for inputting said optical pulse train
sent through said transmission line from this signal sending unit;
the sweeping unit for sweeping the center frequency of this band
pass filter by using a predetermined sweep frequency; the
photoelectric conversion element for receiving the light with the
wavelength component passed through said band pass filter; the
clock extraction module for inputting the electric signal converted
by this photoelectric conversion element to extract the clock
signal synchronized with the transmitted digital data; a feedback
voltage signal outputting unit for outputting a feedback voltage
signal of a voltage in proportion to a time differential of a phase
variation component of this clock signal by inputting the clock
signal extracted by this clock extraction module; and the operation
means for measuring a component of said sweep frequency in the
feedback voltage signal output by this feedback voltage signal
outputting unit to operate chirping of the optical pulse or the
dispersion of said transmission line.
15. The in-service optical pulse evaluation device according to
claim 14, wherein said feedback voltage signal outputting circuit
is a PLL circuit and a cutoff frequency of a low pass filter
constituting this circuit is defined as the frequency higher than
said sweep frequency.
16. The in-service optical pulse evaluation device according to
claim 14, wherein said operation means is a personal computer.
17. The in-service optical pulse evaluation device comprises: the
optical modulation unit for modulating the light ray, which is
emitted from the light source, by using the digital data signal
synchronized with the predetermined clock; a transmission unit for
sending the optical pulse train, which is obtained by modulation of
the optical modulation unit, to the transmission line having a
predetermined dispersion value; a tunable dispersion compensator
for adjusting a specified dispersion value by inputting the optical
pulse train through the band pass filter for inputting the optical
pulse train sent through said transmission line; and a receiver for
receiving the optical pulse train sent through the tunable
dispersion compensator and monitoring the dispersion value to
feedback the result as the specified dispersion value of said
tunable dispersion compensator.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an optical pulse evaluation
device for evaluating a waveform of an optical pulse and an
in-service optical pulse evaluation device, more specifically
relates to the optical pulse evaluation device for evaluating a
pulse waveform expressing an optical intensity of the optical
pulse, an instantaneous frequency of the optical pulse, or a
modulated light prepared by modulating the optical pulse in a light
source end and for observing a waveform change after a known
optical pulse is passed through a device such as an optical fiber
to evaluate a waveform deterioration or a compensation behavior
caused by the device and an in-service optical pulse evaluation
device capable of measuring a wave length dispersion in an optical
communication.
BACKGROUND OF THE INVENTION
[0002] In recent years, a semiconductor laser for outputting a
laser light is increasingly developed and, also, an optical
transmission rate is increasing remarkably. At the same time, in a
region between relatively high bit-rates, in which the optical
transmission rate becomes a range form 125 Mbit/s (megabits/second)
to 40 G (giga) or more bits/s, the optical pulse is increasingly
used in a variety of optical communication technologies.
Particularly, an increase in a communication using a high bit-rate
of 40 G or more bits/s in the future is expected. In the high
bit-rate region, an optical pulse width becomes a ps (pico second
10.sup.-12) or shorter order.
[0003] As a rule, the optical fiber for sending the optical pulse
produces group velocity dispersion, self phase modulation, and
polarization mode dispersion to cause the waveform deterioration of
the optical pulse. A higher transmission rate of the optical pulse
causes a short pulse width resulting in a serious effect of such
the waveform deterioration the optical pulse on a signal
processing.
[0004] The optical pulse evaluation device can evaluate a form of
an output waveform and a presence and absence of jitter for the
optical pulse having a variety of bit-rates or the optical pulse of
the high bit-rate, which are output from an optical pulse
generating light source. On the other hand, for a variety of
optical components and optical devices such as the optical fiber
used in an optical network transmission system, the optical pulse
evaluation device evaluates wavelength dispersion, polarization
mode dispersion, higher dispersion, characteristic of such the
optical pulse to contribute to developments of optical
communication systems and developments of optical components.
[0005] On the other hand, conventionally, the optical pulse
evaluation device widely uses an approach for evaluation of the
optical pulse by using a nonlinear optical effect. Where, the
nonlinear optical effect is a phenomenon in which a relation
between an electromagnetic field of a laser light and electronic
polarization of a matter becomes nonlinear. Methods for evaluation
of the optical pulse by using the nonlinear optical effect include
(1) auto correlation method, (2) cross correlation method, (3) FROF
(frequency resolved optical gating) method, and (4) SPIDER
(spectral phase interferometry for direct electric field
reconstruction) method.
[0006] According to a first proposal of approaches using such the
nonlinear optical effect, a measured optical pulse is distinguished
from the optical pulse produced by four-wave mixing to eliminate a
noise caused by an interference of the measured optical pulse and
the optical pulse produced by four-wave mixing. In this way, the
optical pulse is measured in a high sensitivity and, also, a
spectrogram is divided into two orthogonal light components to
evaluate the optical pulse produced by polarized wave dispersion
(refer to, for example, patent document 1.) According to this first
proposal, the measured optical pulse is divided into a probe light
and a gate light and a two photon absorption intensity is measured
as a function of a delay time and a frequency by using a two photon
transition medium, which is one of nonlinear effects.
[0007] On the other hand, according to a second proposal, a second
harmonic wave of the measured optical pulse is generated by using a
nonlinear optical material and, at the same time, a cross
correlation signal light, which corresponds to a difference
frequency of the measured optical light from the second harmonic
wave, is generated to convert this cross correlation signal light
to an electric signal for displaying its pulse waveform (refer to,
for example, patent document 2.)
[0008] Moreover, according to a third proposal, the measured
optical pulse is divided into 2 optical pulses and these 2 optical
pulses are launched into a nonlinear optical material by making a
delay time and a pulse width and a pulse waveform of the optical
pulse are measured by using an optical pulse intensity waveform,
which is a correlation data with the delay time, of the second
harmonic wave (refer to, for example, patent document 3 and patent
document 4.)
[0009] FIG. 1 shows a configuration of the optical pulse evaluation
device according to this third proposal. This figure expresses the
outline of the device disclosed in patent document 3. Measured
light 11 is launched into interferometer 12 to go to an auto
correlation signal detection unit 13. Interferometer 12 divides
measured light 11 into 2 optical pulses 16 and 17 by using first
and second parallel flat plates 14 and 15 to make an optical path
difference between them. This optical path difference can be
changed by driving unit 18 housed in interferometer 12. Auto
correlation signal detection unit 13 measures a time correlation
between 2 optical pulses 16 and 17 launched from interferometer 12
to calculate the optical pulse width of measured light 11.
[0010] Patent document 4 in the third proposal is same as patent
document 3 in the point that measured light 11 is divided to delay
the one optical pulse and, then, the correlated signals are
measured after convoluting. However, in this document, a wavelength
of the optical pulse is dispersed before measured light 11 is
launched. In conclusion, patent document 4 uses an approach of
measuring the optical pulse width by applying the auto correlation
method and the nonlinear optical effect.
[0011] According to patent document 4, the optical path difference
between 2 light paths is made by using a Mach-Zehnder
interferometer, these light paths are converted to the electric
signals by a light receiver, and a signal processing is carried out
for each spectrum by using a frequency intensity analyzer to obtain
an auto correlation (refer to, for example, patent document 5.)
[0012] FIG. 2 shows the configuration of the optical pulse
evaluation device according to the fourth proposal. The light
emitted from light source 21 is divided by first half mirror 23.
The light passed through first half mirror 23 produces the
predetermined frequency difference by frequency converter 24 and
passes through second half mirror 25 to be input to light receiver
26. The light reflected by first half mirror 23 is delayed by
photodelay device 27 followed by reflection by second half mirror
25 to be input in light receiver 26. Light receiver 26 receives 2
lights, of which frequencies and arrival times differ from each
other, to convert to the electric signals finally followed by
subjecting to the signal processing for each spectrum by using
frequency intensity analyzer 28 located in its output end.
[0013] Patent document 1: Japanese Published Unexamined Patent
Application No. 2003-28724 (paragraph 0041, FIG. 21)
[0014] Patent document 2: Japanese Published Unexamined Patent
Application No. 2002-257633 (paragraph 0025, FIG. 2)
[0015] Patent document 3: Japanese Published Unexamined Patent
Application No. 2001-74560 (paragraph 0081, FIG. 2)
[0016] Patent document 4: Japanese Published Unexamined Patent
Application No. 2000-356555 (paragraph 0010, FIG. 1)
[0017] Patent document 5: Japanese Published Unexamined Patent
Application No. 1997-133585 (paragraph 0207, FIG. 1)
[0018] In general, carrying out a characteristics evaluation of the
pulse width and the pulse waveform of the optical pulse of the
optical pulse by using the nonlinear optical effect arises the
problem in that a sensitivity of the measurement is low and
accuracy of measurement is limitedly improved. According to the
first proposal, the optical pulse is evaluated by applying the
nonlinear optical effect and, thus, accuracy of measurement is
limited depending on intensity of an incident electric field.
Therefore, the sensitivity of measurement cannot be increased to
disturb the characteristics evaluation of the optical pulse in use
of the light source having a low power. On the other hand, this
proposal arises the problem in that when evaluation of the optical
pulse used for high transmission velocity with a high bit-rate is
carried out, a high time resolution performance such as pico second
or shorter is required as the pulse width of the optical pulse to
be evaluated becomes short.
[0019] According to the second proposal, the second harmonic wave
of the sampling light is generated and, at the same time, a
material called pseudo phase matching element is necessarily used
through a special procedure for generating a difference frequency
light as a cross correlation signal light between the generated
second harmonic wave and the measured light. In other words, the
second proposal has a constraint for commercialization, if no
nonlinear optical material is available to satisfy a phase matching
condition for the wavelength of the measured light. On the other
hand, this proposal uses also the nonlinear optical effect for
evaluation of the optical pulse. Therefore, the sensitivity of the
measurement is low to make the characteristics evaluation of the
optical pulse inappropriate in use of the light source providing
the low power.
[0020] The third proposal is same as the first and the second
proposals in the point of using the nonlinear optical effect.
Hence, measurement depends on the incident electric field to make
the sensitivity of the measurement low and, thus, to make the
characteristics evaluation of the optical pulse inappropriate in
use of the light source providing the low power.
[0021] According to the fourth proposal, phase information becomes
available by measuring the auto correlation of electric field
components of the measured optical pulse and, thus, the auto
correlation method is realized allowing a high relative sensitivity
and almost no limitation of a measurable wavelength range in a low
light intensity. However, according to the fourth proposal, the
circuit configuration and a circuit control of frequency intensity
analyzer 28 shown in FIG. 2 is complicated and, hence, the
measurement can be unstably carried out for a position and a
frequency drift of the optical pulse. Therefore, the proposal is
inappropriate for evaluating the optical pulse for high velocity
transmission, which has a relatively high bit-rate and a small
pulse width, highly desired for the characteristics evaluation.
[0022] On the other hand, any optical pulse evaluation device
cannot observe a spectral intensity to each spectral phase in a
region of the optical pulse, which has a relatively high bit-rate,
namely, cannot observe an actual pulse waveform.
[0023] Where, the spectral phase will be additionally described
below. When a complex notation of an electric field spectrum of the
optical pulse is denoted by E(.omega.), this can be expressed by
the following formula:
E(.omega.)=.vertline.E(.omega.).vertline.exp [i.phi.(.omega.)]
(1)
[0024] As known from this formula, the complex notation E(.omega.)
of the electric field spectrum of the pulse can be expressed by a
amplitude .vertline.E(.omega.).vertline. and an argument
.phi.(.omega.). This argument .phi.(.omega.) is named the spectral
phase.
[0025] Consequently, in above described devices conventionally
proposed, the intensity of the optical pulse and the pulse width
based on a delay processing to be evaluated are obtained. In such
the conventional optical pulse evaluation device, the actual pulse
waveform of the optical pulse is presumed by applying information
obtained about the optical pulse to the form of a standard pulse
waveform for example, an eight and a width of a Gaussian waveform.
Such the characteristics evaluation does not allow showing an
actual distortion of a waveform to make high accuracy evaluation
impossible for a deterioration behavior of the waveform of the
optical device itself or the waveform, which are caused by such the
optical device as the optical fiber.
[0026] So far, the deterioration behavior of the waveform of the
optical device itself or the waveform, which are caused by such the
optical device as the optical fiber, cannot be highly precisely
evaluated in a state, where a data communication is operated.
SUMMARY OF THE INVENTION
[0027] The object of the present invention is to provide the
optical pulse evaluation device capable of the characteristics
evaluation such as the evaluation of the optical pulse itself or
the evaluation of the change of the spectral intensity and spectral
phase induced by optical devices or samples in which the optical
pulse propagates, i.e. the spectral response of optical devices,
and an in-service optical pulse evaluation device for measuring the
wave length dispersion in the optical communication operated at
relatively high bit-rate.
[0028] According to the present invention, the optical pulse
evaluation device comprises (a) an optical pulse outputting means
for outputting an optical pulse to be evaluated, (b) a optical
frequency component extracting means for extracting a specific
optical frequency component of the optical pulse output from this
optical pulse outputting means, (c) a frequency component intensity
measurement means for measuring intensity of the specific optical
frequency component of the optical pulse extracted by this optical
frequency component extracting means, and (d) a phase intensity
operating means for operating the spectral phase and the spectral
intensity of the optical pulse output from the optical pulse output
means on the basis of a measurement result by this frequency
component intensity measurement means.
[0029] According to the present invention, the specific optical
frequency component of the optical pulse output from the optical
pulse outputting means is extracted by the optical frequency
component extracting means and the spectral phase and the spectral
intensity of the optical pulse are operated by using an extracted
frequency and the intensity of the frequency component of the
optical pulse, which are the measurement results by the frequency
component intensity measurement means.
[0030] Further according to the present invention, the optical
pulse evaluation device comprises (a) the optical pulse outputting
means for outputting an optical pulse to be evaluated, (b) an
optical dividing means for dividing the optical pulse output from
this optical pulse outputting means, (c) the optical frequency
component extracting means for extracting the specific optical
frequency component by receiving the one of optical pulses divided
by this optical dividing means, (d) the frequency component
intensity measurement means for measuring intensity of the specific
optical frequency component of the optical pulse extracted by this
optical frequency component extracting means, (e) a whole optical
intensity measurement means for measuring the intensity of a whole
optical pulse of the other divided by the optical dividing means,
and (f) the phase intensity operating means for operating the
spectral phase and the spectral intensity of the optical pulse
output from the optical pulse output means on the basis of the
measurement result of the frequency component intensity measurement
means and the whole optical intensity measurement means.
[0031] According to the present invention, the optical pulse output
from the optical pulse outputting means is divided by the optical
dividing means and the divided one is used for operating the
spectral phase and the spectral intensity of the optical pulse in
the same was as that of the invention described in claim 1.
According to the present invention, the other optical pulse output
the optical dividing means is also used for the operation and,
thus, using this as a reference signal allows canceling out a drift
of a pulse position and the drift of the frequency, when these
occur.
[0032] Further according to the present invention, an in-service
optical pulse evaluation device comprises (a) an optical modulation
unit for modulating a light ray, which is emitted from a light
source, by using a digital data signal synchronized with a
predetermined clock, (b) an optical divider for dividing an optical
pulse train, which is obtained by modulation of this optical
modulation unit, in a first and a second paths by using a
transmission line, (c) a band pass filter for inputting the optical
pulse train obtained by modulation of this optical modulation unit
located in the first path, (d) a sweeping unit for sweeping a
center frequency of this band pass filter, (e) a first
photoelectric conversion element for receiving the light with the
wavelength component passed through the band pass filter, (f) a
first clock extraction module for inputting the electric signal
converted by this first photoelectric conversion element to extract
a clock signal synchronized with the digital data transmitted, (g)
a second photoelectric conversion element for receiving the optical
pulse train transmitted through the second path, (h) a second clock
extraction module for inputting the electric signal converted by
this second photoelectric conversion element to extract the clock
signal synchronized with the digital data transmitted, (i) a phase
detection means for detecting the phase of the clock signal output
by the first and the second clock extraction modules to calculate
time delayed by the band pass filter, and (j) an operation means
for measuring the measurement result by the phase detection means
during sweeping the center frequency of the band pass filter by the
sweeping unit to operate chirping of the optical pulse or a
dispersion of the transmission line.
[0033] According to the present invention, the light ray is
modulated by using the digital data signal synchronized with the
predetermined clock to make transmission through the transmission
line divided in 2 paths. In the first path of these 2 paths, the
light with the wavelength component passed through the band pass
filter is converted to the electric signal by the first
photoelectric conversion element to extract the clock signal
synchronized with the transmitted digital data by the first clock
extraction module. In the second path, the clock signal
synchronized with the transmitted digital data is extracted by the
second clock extraction module without passing through the band
pass filter. The phase detection means detects the phase of the
clock signal output by the first and the second clock extraction
modules to calculate time delayed by the band pass filter. Finally,
when the sweeping unit sweeps the center frequency of the band pass
filter, the result of detection by the phase detection means is
measured for operation of chirping of the optical pulse or the
dispersion of the transmission line by the operation means.
[0034] Further according to the present invention, the in-service
optical pulse evaluation device comprises (a) the optical
modulation unit for modulating the light ray, which is emitted from
the light source, by using the digital data signal synchronized
with the predetermined clock, (b) a signal sending unit for sending
the optical pulse train, which is obtained by modulation of the
optical modulation unit, to the transmission line, (c) the band
pass filter for inputting the optical pulse train sent through the
transmission line from this signal sending unit, (d) the sweeping
unit for sweeping the center frequency of this band pass filter by
using a predetermined sweep frequency, (e) the photoelectric
conversion element for receiving the light with the wavelength
component passed through the band pass filter, (f) the clock
extraction module for inputting the electric signal converted by
this photoelectric conversion element to extract the clock signal
synchronized with the transmitted digital data, (g) a feedback
voltage signal outputting unit for outputting a feedback voltage
signal of a voltage in proportion to a time differential of a phase
variation component of this clock signal by inputting the clock
signal extracted by this clock extraction module, and (h) the
operation means for measuring a component of the sweep frequency in
the feedback voltage signal output by this feedback voltage signal
outputting unit to operate chirping of the optical pulse or the
dispersion of the transmission line.
[0035] According to the present invention, the optical pulse train
generated by modulation of the light ray by the digital data signal
synchronized with the predetermined clock is sent to the
transmission line and, in a receiving end, the received optical
pulse train is passed through the band pass filter, in which the
center frequency is swept at the sweep frequency, and is converted
to the electric signal with the photoelectric conversion element.
The original clock signal is extracted from this electric signal
and feedback voltage signal outputting unit outputs the feedback
voltage signal of the voltage in proportion to the time
differential of the phase variation component of the clock signal.
Through measuring the component of the sweep frequency in this
feedback voltage signal, operation is carried out for chirping of
the optical pulse or the dispersion of the transmission line.
[0036] Further according to the present invention, the in-service
optical pulse evaluation device comprises (a) the optical
modulation unit for modulating the light ray, which is emitted from
the light source, by using the digital data signal synchronized
with the predetermined clock, (b) a transmission unit for sending
the optical pulse train, which is obtained by modulation of the
optical modulation unit, to the transmission line having a
predetermined dispersion value, (c) a tunable dispersion
compensator for adjusting a specified dispersion value by inputting
the optical pulse train through the band pass filter for inputting
the optical pulse train sent through the transmission line, and (d)
a receiver for receiving the optical pulse train sent through the
tunable dispersion compensator and monitoring the dispersion value
to feedback the result as the specified dispersion value of the
tunable dispersion compensator.
[0037] According to the present invention, the optical pulse train
generated by modulation of the light ray by the digital data signal
synchronized with the predetermined clock is sent to the
transmission line and, in a receiving end, the dispersion value is
monitored to send to the tunable dispersion compensator for
feedback control thereof in order to adjust the specified
dispersion value.
[0038] As described above, according to the present invention, the
spectrum of the optical pulse can be measured in the region having
the relatively high bit-rate without deterioration of the signal
and, hence, deterioration of the waveform can be analyzed in detail
when the optical pulse is passed through the sample. Consequently,
when the optical fiber system is built up, for example, measuring
and evaluating highly precisely deterioration of the signal caused
by light propagation to compensate the dispersion allows a light
propagation distance to increase and an occurrence of a signal
error to reduce. In addition, according to the present invention,
when a repetition frequency becomes higher and the pulse width
becomes narrower, the optical spectrum becomes relatively wider
and, thus, as a rule, the characteristics evaluation of the optical
pulse can be advantageously carried out in the relatively high
bit-rate, which is difficult to allow analyzing the optical
pulse.
[0039] On the other hand, according to the present invention, in
the state where the data communication is actually working, the
characteristics evaluation of the optical pulse can be carried out
in the relatively high bit-rate and, therefore, it is unnecessary
to stop temporarily a data transmission service for the
characteristics evaluation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 is a schematic block diagram showing a constitution
of an in-service optical pulse evaluation device according to the
third proposal.
[0041] FIG. 2 is the schematic block diagram showing the
constitution of the in-service optical pulse evaluation device
according to the fourth proposal.
[0042] FIG. 3 is the schematic block diagram showing the
constitution of an optical pulse evaluation device of the first
example according to the present invention.
[0043] FIG. 4 is a characteristic view showing a result of
measurement of the optical pulse in the first example.
[0044] FIG. 5 is a block diagram of a main part of the constitution
of an optical pulse evaluation device of a first modification
example of the first example according to the present
invention.
[0045] FIG. 6 is the characteristic view of the optical pulse from
a wide bandwidth light source in the first modification
example.
[0046] FIG. 7 is a figure showing a result of a simulation in a
second modification example of the first example according to the
present invention.
[0047] FIG. 8 is a flow chart of an operation processing for
compensation of a filter characteristic in the second modification
example of the first example according to the present
invention.
[0048] FIG. 9 is the schematic block diagram showing the optical
pulse evaluation device used for sample evaluation in a third
modification example of the first example according to the present
invention.
[0049] FIG. 10 is a waveform chart showing a waveform of a
regularly arriving optical pulse and an intermittently arriving
optical pulse.
[0050] FIG. 11 is the schematic block diagram showing the main part
of the in-service optical pulse evaluation device according to the
second example according to the present invention.
[0051] FIG. 12 is the schematic block diagram showing the
in-service optical pulse evaluation device of the third example
according to the present invention.
[0052] FIG. 13 is a view illustrating a sweeping behavior of a
tunable wavelength optical band pass filter according to the third
example.
[0053] FIG. 14 is a system block diagram of a communication system
of the first experimental example.
[0054] FIG. 15 is the waveform chart showing a signal spectrum of
each unit of the communication system according to the first
experimental example shown in FIG. 14.
[0055] FIG. 16 is the waveform chart showing a waveform example of
a phase modulation component detected by a PLL circuit in
accordance with time passed in the first experimental example.
[0056] FIG. 17 is the figure showing a characteristic of a relation
between a dispersion value of an optical fiber and a feedback
voltage in the first experimental example.
[0057] FIG. 18 is the system block diagram of the communication
system used in the second experimental example.
[0058] FIG. 19 is the figure showing the characteristic of the
example of measurement of a transient response of a tunable
dispersion compensator in the second experimental example.
[0059] FIG. 20 is the system block diagram of an outline of the
communication system used in the third experimental example.
[0060] FIG. 21 is the figure showing the characteristic of result
of measurement of a bit error rate in the third experimental
example.
[0061] FIG. 22 is the figure showing the characteristic of a
temporal change of the dispersion value and an error count resulted
from an adaptive dispersion compensation experiment in the third
experimental example.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
EXAMPLE 1
[0062] FIG. 3 shows a constitution of an optical pulse evaluation
device of the first example according to the present invention.
Optical pulse evaluation device 41 has optical pulse light source
43 outputs repeatedly optical pulse 42, which is evaluated in the
present example, at a predetermined repetition frequency fREP. The
repetition frequency fREP of optical pulse 42 is indicated on
optical pulse light source 43 by repetition frequency defining
means 44. A defined repetition frequency fREP is also input in
phase detection circuit 45.
[0063] In this way, the optical pulse 42 expressed by the complex
notation E(.omega.).vertline. and the phase .phi.(.omega.) output
from optical pulse light source 43 at the defined repetition
frequency fREP is input in tunable wavelength optical band pass
filter 47. Where, ".omega." represents an angular frequency.
Tunable wavelength optical band pass filter 47 inputs tunable
wavelength-directing signal 49 for changing the wavelength, which
is passed through, from stage 48. By this step, the wavelength
component 50 of a wavelength region defined for optical pulse 42 is
only passed. The wavelength component 50 passed through tunable
wavelength optical band pass filter 47 is emitted to photo diode
(PD) 51. If it is assumed that optical pulse 42 is output from
optical pulse light source 43 at relatively high repetition
frequency fREP such as some giga to some ten giga bits/second, a
velocity of changing the passed wavelength of tunable wavelength
optical band pass filter 47 by stage 48 using tunable
wavelength-directing signal 49 is the velocity relatively slow
exemplified by 0.22 nm/s (nanometer/second.) This is because a
plurality of optical pulses 42 output repeatedly from optical pulse
light source 43 is measured for the identical passed wavelength to
calculate an average value thereof to keep reliability of the
measurement for each wavelength.
[0064] The output of photo diode 51 is input in phase detection
circuit 45. Phase detection circuit 45 according to the present
example is constituted by a lock-in amplifier and operates
frequency conversion of a measured signal to a direct current by
using a heterodyne detection technology. According to the present
example, repetition frequency fREP output from repetition frequency
defining means 44 is input as a lock-in signal (LO) and the output
of photo diode 51 is input as a reference signal (RF) The lock-in
amplifier operates heterodyne detection of a lock-in signal as a
local signal to detect the phase. In this step, positional
information as a sine(sin) component of the wavelength component
passed through tunable wavelength optical band pass filter 47 and
amplitude information as a cosine (cos) component are known.
[0065] Such the detection result of phase detection circuit 45 is
input in personal computer 52. Personal computer 52 has a CPU
(central processing unit) not illustrated and a memory medium, in
which a control program for working optical pulse evaluation device
41 is stored, not illustrated. On the other hand, such input
apparatus 53 as a keyboard and a mouse and such output apparatus 54
as a liquid crystal display or a printer are connected to personal
computer 52. Personal computer 52 has operation processing unit 56
for operating a processing for inputting a variety of operation
information to realize optical pulse evaluation device 41 by
executing the stored control program from input apparatus 53,
display processing unit 57 for operating the processing for
displaying the yielded result on output apparatus 54, and operation
unit 58 for realizing a predetermined operation such as an inverse
Fourier transform by software processing.
[0066] Operation processing unit 56 outputs direction signal 61 for
varying the wavelength for stage 48. Operation unit 58 subjects the
signal obtained from phase detection circuit 45 to the inverse
Fourier transform and calculates the amplitude for each phase of
optical pulse 42 to feed to display processing unit 57. Display
processing unit 57 displays the waveform of optical pulse 42 output
from optical pulse light source 43 according to a displayed content
directed by operation processing unit 56.
[0067] FIG. 4 shows the result of measurement of the optical pulse.
In this example, optical pulse light source 43 shown in FIG. 3 uses
a mode-locked laser diode and shows an optical pulse characteristic
in the case where optical pulse 42 is output at the repetition
frequency fREP of 10 gigabits/second. Where, diffraction
grating-type band pass filter of 0.6 nm is used as tunable
wavelength optical band pass filter 47. This tunable wavelength
optical band pass filter 47 can vary the wavelength from 1520 nm to
1600 nm and can change the center frequency by changing its
incident angle. As phase detection circuit 45, a lock-in amplifier
detector is used, the output signal is fed into personal computer
52, and, finally, auto correlation can be obtained for the electric
field of the optical spectrum by operation processing.
[0068] The solid line in FIG. 4(a) represents the signal spectrum
of the measured optical pulse and the broken line represents group
delay (-d.phi./d.omega.) characteristic depending on the
wavelength. The solid line in FIG. 4(b) represents waveform of the
intensity of the optical pulse on a time base and the broken line
represents the optical frequency component. The optical pulse
contains various light wavelengths. In the figure, the left and
right ends of the waveform of the optical pulse shows the presence
of the light having short and long wavelengths as the measurement
result. A pulse width (.tau.) is 2.31 ps in FWHM (Full Width Half
Maximum.) The Fig. (c) shows the waveform expressing the power of
the optical pulse on the time base and the axis of ordinate shows
dB indication. In comparison with the conventional example,
remarkably high accuracy evaluation of the optical pulse waveform
is realized to enable to make indication logarithmically. It can be
known that a dynamic range exceeds 49 dB. By this, a peak with a
small waveform, which cannot be so far observed because lost in the
noise, can be satisfactorily detected in the first example. On the
other hand, a fine form of the pulse waveform can be specified.
[0069] The solid line in FIG. 4(d) represents an auto correlation
waveform produced by using SHG (Second Harmonic Generation) light,
which is one of conventional evaluation methods. In contrast to
this, the circle represents the auto correlation waveform produced
by calculation of the intensity waveform of the figure (c) of the
present example. Hence, it can be known that the present example
shows an auto correlation trace and the pulse width, which are
similar to the conventional evaluation methods. By this,
reliability of an evaluated value itself using the present example
is confirmed.
FIRST MODIFICATION EXAMPLE OF THE FIRST EXAMPLE
[0070] FIG. 5 shows the main part of the optical pulse evaluation
device in the first modification example of the first example
according to the present invention. In FIG. 5, a same reference
numeral is attached to the part identical with that of FIG. 3 and,
thus, description of the part is omitted properly. In the optical
pulse evaluation device 41A of this modified example, optical pulse
42 output from optical pulse light source 43 is divided in 2 parts
by optical divider 71 optical pulse 42A propagating in the line
branched upward in the figure is input in tunable wavelength
optical band pass filter 47 having a 0.6 nm full width half
maximum, similar to the previous example, the light of the
wavelength component passed through this filter is received by
photodiode 51, and its output is input as the reference signal (RF)
of phase detection circuit 45A. Optical pulse 42B propagating in
the line branched downward in the figure is received by the other
photodiode 72 having the equal characteristic to that of photodiode
51 without any limitation of a passband, and its outputs is input
in phase detection circuit 45A as the lock-in signal (LO.) A
section of the circuit other than this is identical to that of the
previous example and, thus, illustration of the part is
omitted.
[0071] In optical pulse evaluation device 41A of this first
modified example, optical pulse 42B obtained by branching of
optical pulse 42 output actually from optical pulse light source 43
is subjected to photoelectric conversion for inputting phase
detection circuit 45A. Therefore, differ from optical pulse
evaluation device 41 of the previous example, in the case where the
position of the optical pulse output from optical pulse light
source 43 is varied by vibration of a component such as a mirror of
an optical system not illustrated and a frequency drifts finely,
the phase detection circuit works with a drifted value as a
reference. As the result, the effect of the drift is canceled to
allow carrying out the evaluation of the optical pulse.
[0072] FIG. 6 shows the result of the evaluation of the optical
pulse by using a super continuum light spreading to the wide
bandwidth using the optical pulse evaluation device of the first
modification example. The figure (a) represents the signal spectrum
of the measured optical pulse and the broken line represents group
delay characteristic as a function of the wavelength. The solid
line in FIG. 4(b) represents the waveform of the intensity of the
optical pulse as a function of time and the broken line represents
the optical frequency component. In the figure (c,) shows the
waveform of the optical pulse on the time base and the axis of
ordinate shows dB indication. Also in this case, as described in
FIG. 4, in comparison with the conventional example, remarkably
high accuracy evaluation of the optical pulse waveform is realized.
It can be known that the dynamic range exceeds 46 dB. By this, the
peak with the small waveform, which could not be so far observed
because of lost in the noise, can be detected. This is called an
optical wave-breaking phenomenon caused by the auto phase
modulation being one of the nonlinear effect. Using optical pulse
evaluation device 41A capable of the high accuracy measurement
enables an observation. In addition to this, the evaluation of a
transmittance characteristic of a saturable absorber becomes
possible.
[0073] The solid line in the figure (d) shows the auto correlation
waveform produced by using SHG (Second Harmonic Generation) light,
which is one of conventional evaluation methods. The circle
represents the auto correlation waveform produced by calculation of
the intensity waveform of the figure (c) of the present example.
Hence, it has been known that the present modified example shows
also the auto correlation trace and the pulse width, which are
similar to the conventional evaluation methods. By this, the
reliability of the evaluated value itself is confirmed. On the
other hand, the waveform presented in this figure (d) differs from
the actual waveform of the optical pulse to be evaluated. This
point is same as that of the figure (d.)
SECOND MODIFICATION EXAMPLE OF THE FIRST EXAMPLE
[0074] In the second modification example of the first example
according to the present invention, it is same as that of the first
modified example that optical pulse evaluation device 41 shown in
FIG. 3 is used and, however, the control program to be installed in
personal computer 52 differs slightly from the previous example in
the point that the optical intensity waveform and the instantaneous
frequency can be obtained by reconstructing the measurement result
of the optical pulse.
[0075] In the second modification example, a filter characteristic
of tunable wavelength optical band pass filter 47 shown in FIG. 3
is assigned to Lorenz type (Cauchy distribution), which is common
filter characteristics as seen in conventional dielectric
multilayer band pass filters, and the filter bandwidth is defined
as 0.6 nm. On the other hand, optical pulse 42 output from optical
pulse light source 43 is assigned to the pulse having a Gaussian
waveform having 1 picosecond full width half maximum and the
repetition frequency fREP of 10 gigabits/s. In addition, in the
second modification example, what the spectrum intensity and the
spectrum phase are calculated from the amplitude and the phase of a
photoelectrically converted photocurrent output from photodiode 51
is same as that of the example. Such information is reconstructed
by personal computer 52 to know the optical intensity waveform and
the instantaneous frequency.
[0076] FIG. 7 shows the result of the simulation in the second
modification example according to the present invention. The figure
(a) shows the waveform and the instantaneous frequency as a change
of the frequency chirp in the case where the filter characteristics
is not compensated for. Where, the "chirp" is the proportion of the
change of the instantaneous frequency and also named a frequency
chirp or chirping. In the figure (a,) a distortion of the pulse
waveform is caused by the dispersion of the filter. In contrast to
this, in the figure (b,) an arithmetic operation is executed for
compensating the filter characteristics to eliminate the distortion
of the pulse waveform and the instantaneous frequency.
[0077] FIG. 8 shows the flow of the operation processing for
compensating the filter characteristics in this second modification
example. Where, when an optical filter characteristics function
.zeta.(.omega.) is calculated by using personal computer 52 shown
in FIG. 3 (step S81,) a delay part of tunable wavelength optical
band pass filter 47 for optical pulse 42 has been included into the
transfer function of the optical filter H. And, a measured
photocurrent phasor .vertline.(.omega.) is deconvoluted with
optical filter characteristics function .zeta.(.omega.) (step S28)
to know the complex notation E(.omega.) of the measured pulse (step
S83) and to know a time wavelength e(t) of the measured pulse (step
S84.)
THIRD MODIFICATION EXAMPLE OF THE FIRST EXAMPLE
[0078] In the first example and the modified example as described
above, characteristics of the optical pulse itself output from the
optical pulse light source were evaluated. A known optical pulse
can be used for measuring and evaluating a variety of optical
devices.
[0079] FIG. 9 shows the outline of the constitution of the optical
pulse evaluation device for evaluating the sample, as the third
modification example of the first example according to the present
invention. In FIG. 9, the same reference numeral is attached to the
part identical with that of FIG. 3 and, thus, description of the
part is omitted properly. In optical pulse evaluation device 41C of
this modification example, a movable sample stage 91 is located
between optical pulse light source 43 and tunable wavelength
optical band pass filter 47 of optical pulse evaluation device 41,
which are shown in FIG. 3. Sample stage 91 can be moved in arrow 92
crossing the light path to make it attachable to and detachable
from the light path. As illustrated, when sample stage 91 is
located in the light path, the light passed through sample 93
mounted on sample stage 91 is launched to tunable wavelength
optical band pass filter 47. On the other hand, in the state where
sample stage 91 moves to a position out of the light path, optical
pulse 42 output from optical pulse light source 43 is intactly
launched to tunable wavelength optical band pass filter 47. In
sample stage 91, a driving mechanism not illustrated controls the
movement in arrow 92 direction by a direction of operation
processing unit 56C in personal computer 52.
[0080] In optical pulse evaluation device 41C having such the
constitution, in the state, for example, where sample 93 such as
the optical fiber is not inserted in the light path between optical
pulse light source 43 and tunable wavelength optical band pass
filter 47, characteristics of optical pulse 42 itself are measured
in the same way as that of FIG. 3. Next, operation processing unit
56C gives directions to sample stage 91 on putting sample 93 in the
light path and, in the state, the same measurement is performed.
Operation unit 58C knows characteristics of sample 93 itself to the
optical pulse as the reference. The directions for characteristics
to be known are issued to personal computer 52 by input apparatus
53. By this, the chirping, dispersion, and phase shift of a
reference optical pulse are measured and a display result is output
to output apparatus 54 by display processing unit 57.
[0081] Where, the reference optical pulse is the optical pulse
reproduced from an ideal optical pulse or the optical pulse
generated under an environmental condition. Optical pulse 42 itself
output from optical pulse light source 43 has an individuality and,
thus, is not the ideal optical pulse or the optical pulse
reproduced from a situation. Therefore, once the characteristics of
optical pulse 42 output actually from optical pulse light source 43
in the state, where sample 92 is absent in the light path, is
measured and, on the basis of this, the characteristics of the
optical pulse after passed through sample 93 is corrected to enable
to evaluate sample 93 under a variety of environmental
conditions.
[0082] It has been described above that in optical pulse evaluation
device 41C of the third modification example, sample stage 91 is
automatically attached to and detached from the light path.
However, it may be sufficient that an operator mounts manually
sample stage 91 on the light path and unmounts it manually from the
light path. On the other hand, it is not necessary that the optical
pulse evaluation device for evaluating the sample uses optical
pulse evaluation device 41 shown in FIG. 3 as a base and, for
example, optical pulse evaluation device 41A shown in FIG. 5 may be
used.
[0083] In the first example and modification examples as described
above, any special description is not given to elimination of
errors caused by dispersion, or transmittance characteristic, or
delay characteristic of tunable wavelength optical band pass filter
47. However, such errors can be eliminated by the arithmetic
operation based on a known method already proposed. By this, not
only the dielectric multilayer band pass filter preferable as
tunable wavelength optical band pass filter 47 but also the optical
band pass filter using the diffraction grating system and the
tunable wavelength fiber Bragg grating system allows the known
method to eliminate errors, which is caused by these factors, to
measure highly precisely. For example, in the case where the
dielectric multilayer is used as tunable wavelength optical band
pass filter 47, it is attempted to pass actually the optical pulse
to measure the delay time. Next, the value is stored in personal
computer 52 as data for correction and, then, used for correction
of errors.
[0084] In the first example, the pulse train generated in a
predetermined repetition cycle is evaluated by using the optical
pulse evaluation device. However, the object of the evaluation is
not limited to this. The present invention can be applied to all
the pulse light sources and be used for various things and matters
such as mode locking and gain-switching.
EXAMPLE 2
[0085] When as described above, the transmission velocity of the
optical communication system becomes higher resulting in the
communication in the bit rare over 40 Gbit/s, the distortion of the
waveform, which is caused by the chromatic dispersion of an optical
devise located in the optical fiber and the transmission line,
becomes remarkable. Where, the chromatic dispersion is a
phenomenon, in which the propagation velocity (group velocity) of
the light differs from each other in accordance with the
wavelength. If the chromatic dispersion is not zero, a propagation
delay time differs between individual spectrum components of the
optical pulse and, hence, the time width of the optical pulse
(hereafter pulse width) widens. In the optical communication
system, a widened pulse width causes an inter-symbol interference
to make an accurate information transmission impossible. An
allowance of the chromatic dispersion is named a dispersion
tolerance. In an actual optical communication system, a dispersion
value should be administered to make a total dispersion fall in a
dispersion tolerance range. Specifically, the dispersion value of
the transmission line is measured and, by using a dispersion
compensator such as a dispersion compensation fiber having the
dispersion of a code reverse thereto, an accumulated dispersion
should be compensated. Consequently, knowing the dispersion value
of the transmission line is practically very important.
[0086] Knowing the phase or the instantaneous frequency of the
optical pulse enables to know the chirp of the optical pulse. On
the basis of this, in the optical communication system, an amount
of the dispersion received by the optical pulse and the amount of
the dispersion to be compensated are determined. As clearly known
from the first example and modification examples thereof as
described above, the optical pulse arriving in a specific time
interval makes knowing the phase or the instantaneous frequency of
the optical pulse possible.
[0087] However, in the state of in-service where the optical
communication system provides the service, a signal light is
modulated by the digital data. Therefore, optical pulse evaluation
device 41, which is shown in FIG. 3, based on the optical pulse
regularly repeating in the specific time interval cannot be
intactly used in the actual optical communication system.
[0088] FIG. 10 shows 2 optical pulse trains. In this figure, the
axis of abscissa represents the time passed. The figure (a) shows
the optical pulse train, which is output from optical pulse light
source 43 shown in FIG. 3, arriving in the specific time interval.
In contrast to this, the figure (b,) shows the optical pulse train,
which is converted by the digital data, and an example of an actual
transmission signal light in the optical communication. As shown in
the figure (a,) in case of the optical pulse train arriving in the
specific time interval, a photodetector such as the photodiode
shown in FIG. 3 outputs the electric signal having the repetition
frequency synchronized with the optical pulse train. Therefore,
using a phase comparator for the repetition frequency component
provides the delay time of the optical pulse.
[0089] Notwithstanding, as shown in FIG. 10(b,) in case of the
signal light modulated by the digital data, a periodic regularity
becomes absent in the pulse train due to the sent digital data to
cause a lack of a pulse. Therefore, the phase comparator is not
easily usable. The second example according to the present
invention solves such the problem as follows. 1) The
photoelectrically converted electric signal itself is not used, but
the clock signal synchronized with the transmitted digital data is
extracted for use. 2) For making the constitution simple,
improvement is achieved as that the center frequency of optical
band pass filter is swept at a specific repetition frequency and
the chirp is obtained from a phase displacement of the clock
signal.
[0090] FIG. 11 corresponds to FIG. 5 in the first modification
example of the first example and shows the main part of the
in-service optical pulse evaluation device according to the second
example. In FIG. 11, the same reference numeral is attached to the
part identical with that of FIG. 3 and FIG. 5 and, thus,
description of the part is omitted properly.
[0091] In in-service optical pulse evaluation device 141 has pulse
train outputting means 143 for outputting a pulse train 142
modulated by the digital data. Optical pulse light source 43 and
repetition frequency defining means 44 shown in FIG. 5 are absent.
It is assumed that the bit-rate for the pulse train 142 is assigned
to B. Pulse train 142 is divided in 2 parts by optical divider 71.
In the figure, the optical pulse train 142A propagated in the
optical path branched upward in the figure, similar to the previous
example, is input in tunable wavelength optical band pass filter 47
having 0.6 nm full width half maximum, the light of the wavelength
component passed through this filter is received by photodiode 51,
and electric signal 152 as its output is input in first clock
extraction module 161. Reference signal 153 is input from first
clock extraction module 161 to reference signal inputting terminal
(RF) of phase detection circuit 45B.
[0092] Optical pulse train 142B propagating in the light path
branched toward the bottom of the figure is received by the other
photodiode 72 having the equal characteristic to that of photodiode
51 without any limitation of the passband. Its output 154 is input
in second clock extraction module 162. The lock-in signal 155 is
input from second clock extraction module 162 to a lock-in
signal-inputting terminal (LO) of phase detection circuit 45B. The
section of the circuit other than these is identical to that of the
first example and, thus, illustration of the part is omitted.
[0093] For in-service optical pulse evaluation device 141, first, a
synchronized clock of (1) will be described below. In in-service
optical pulse evaluation device 141, the transmission signal light
in the optical communication is assumed as a measured optical
pulse. Optical pulse train 142 has various frequency components
and, hence, as described above, a simple phase comparison is
difficult. Therefore, first and second clock extraction module 161
and 162 extract each clock signal synchronized with the transmitted
digital data (a sinusoidal wave of which the repetition frequency
is B or the digital waveform of which repetition frequency is B.)
This operation called clock extraction or clock recovery can
arbitrarily use an existing method such as using a narrow bandwidth
filter of which the central wavelength is the bit-rate B and using
a phase lock loop circuit. The phase of clock signal is
synchronized with the optical pulse train. Consequently, similarly
to FIG. 4 of the first example, the delay time, which is caused by
tunable wavelength optical band pass filter 47, can be known by
phase comparison and sweeping the center frequency of tunable
wavelength optical band pass filter 47 allows obtaining the
chirping of the optical pulse. In other words, the clock signal
synchronized with the digital data signal is extracted and a phase
change of this clock signal is detected to know the spectral phase
from the phase change of the clock signal, and the chirping of the
optical pulse can be known from the change of the spectral phase
when the center frequency of the band pass filter is swept.
[0094] In in-service optical pulse evaluation device 141, the
optical pulse train (refer to FIG. 10(b)) obtained by modulating
the intensity or the phase of the light by using the digital data
signal is input in tunable wavelength optical band pass filter 47.
Where, the digital data signal contains a pseudo random signal as
an irregular signal repeated temporarily and spatially.
EXAMPLE 3
[0095] Next, "(2) simplification of the constitution is intended by
improving optical pulse evaluation device 41A" will be described
below. The constitution of FIG. 11 is an improved constitution of
FIG. 5. In this case, for making the reference of the phase, pulse
train 142 was branched in 2 paths and clock extraction should be
carried out for each of first and second clock extraction module
161 and 162. In the third example according to the present
invention, in comparison with the second example, simplification of
the constitution of in-service optical pulse evaluation device is
intended.
[0096] FIG. 12 shows the in-service optical pulse evaluation device
of the third example. This in-service optical pulse evaluation
device 141A inputs intactly pulse train 142, which is modulated by
the digital data output from pulse train outputting means 143, in
tunable wavelength optical band pass filter 147. To tunable
wavelength optical band pass filter 147, sinusoidal wave 172 having
a sweep width .DELTA.f and a sweep frequency fr is supplied from
sinusoidal wave generating device 171 to operate periodical
sweeping. Pulse train 173, which is output from tunable wavelength
optical band pass filter 147 after sweeping, is input in clock
extraction module 174 to operate extraction of clock signal 175.
Extracted clock signal 175 is input in phase detection circuit 176
composed of PLL (Phase Locked Loop) circuit. The section of the
circuit other than this is identical to that of the first example
and, thus, illustration of the part is omitted.
[0097] FIG. 13(a) shows a sweeping behavior of the tunable
wavelength optical band pass filter. Curve 181 expresses the
optical spectrum to be measured or the spectrum of an arrived
signal light. Curve 182 expresses the transmittance spectrum of the
light transmitted through tunable wavelength optical band pass
filter 147, which is shown in FIG. 12, to be input in photo diode
51. On the other hand, Curve 183 expresses the waveform of
sinusoidal wave 172 with the sweep width .DELTA.f and the sweep
frequency fr. FIG. 13(b) shows the change of the central system of
tunable wavelength optical band pass filter 147. The figure (c)
shows the change of the phase of clock signal 175 output from clock
extraction module 174 in FIG. 12.
[0098] With reference to FIG. 12, an operation principle of
in-service optical pulse evaluation device 141A of the third
example will be described below. In accordance with the center
frequency of tunable wavelength optical band pass filter 147, the
phase of clock signal 175 changes. Therefore, the phase of clock
signal 175 is also swept by using the sweep frequency fr as shown
in FIG. 13(c.) The amplitude AM of this phase modulation component
proportions to the chirp (i.e., remained dispersion) of the
measured light pulse. Thus, it is sufficient to know this amplitude
AM.
[0099] The difference from FIG. 11 of the second example will be
described below. In FIG. 11, a direct current component of the
clock signal is detected and, on the contrary, in the third
example, an alternate current (frequency fr) component is detected.
Therefore, in the third example, the reference phase is unnecessary
and a phase detector (for example PPL circuit) providing a high
sensitivity to the phase displacement can be used. On the other
hand, as a practical advantage, it is possible to tolerate a large
noise located in low frequency region. In the third example, it is
not possible to determine a direction (sign of dispersion) of the
chirp from only the amplitude of the phase of clock signal 175.
However, the decision is possible by using a phase relation with
the waveform of sinusoidal wave 172 sweeping tunable wavelength
optical band pass filter 147.
FIRST EXPERIMENTAL EXAMPLE
[0100] For in-service optical pulse evaluation device 141A shown in
FIG. 12, measurement capability of the chirp and the dispersion of
the optical pulse is actually examined by using the optical
communication system. Here, in consideration of an practical usage,
the chirp of the optical pulse was not measured, but the dispersion
value received by the optical pulse was measured. If the optical
pulse to be input is in chirp-free (the state of absence of chirp,)
the chirp and the dispersion of the optical pulse after transmitted
is are equal to each other.
[0101] FIG. 14 shows the communication system of the first
experimental example, which is an embodiment of the in-service
optical pulse evaluation device of the third example. Communication
system 200 of the first experimental example has optical pulse
transmitter (Tx) 202 for outputting the pulse train modulated by
the digital data. Optical pulse transmitter (Tx) 202 has light
source 221 and optical modulator 222 for modulating the light
output from this light source 221 by the digital data signal
synchronized with the predetermined clock. Between optical pulse
transmitter 202 and acousto-optic tunable filter (AOTF) 203
constituting the tunable wavelength optical band pass filter,
optical fiber 205 is connected to send the signal light (pulse
train) 204 after modulation by optical modulator 222. To
Acousto-Optic Tunable Filter 203, sinusoidal wave generating device
271 is connected to supply sinusoidal wave 207, of which sweep
frequency fr becomes 10 kHz. The signal light 208 output from
acousto-optic tunable filter 203 after sweeping is input in
photodiode 209 to be converted to the electric signal. Electric
signal 210 output by photodiode 209 is input in clock extraction
module 212 for extracting and outputting clock signal 211.
Extracted clock signal 211 is input in PLL circuit 213 and PLL
output signal 214 is input in AD converter 215. AD converter 215
sends digital data 216 after conversion to personal computer (PC)
217.
[0102] In such communication system 200 of the first experimental
example, a transmission conditions for sending the digital data by
optical pulse transmitter 202 are 40 Gbits/s for the bit-rate B and
1549.65 nm for the wavelength of the signal light. For a
transmission format, RZ (return-to-zero) system is applied.
Acousto-optic tunable filter 203 changes the central wavelength in
a very high speed in tunable wavelength optical band pass filter
147 (FIG. 12) (T. N. Akazawa et al., Technical Digest of Optical
Fiber Communication '98, paper PD1, 1998.) The bandwidth of
acousto-optic tunable filter 203 of the present example is 0.55 nm
(70 GHz.) The sweep frequency fr is 10 kHz and the sweep width
.DELTA.f of acousto-optic tunable filter 203 is 20 GHz. The light
output from acousto-optic tunable filter 203 is subjected to
photoelectric conversion by photodiode 209 and, then, extracts
clock signal 211 of 40 GHz by clock extraction module 212. PLL
output signal 214 output from PLL circuit 213 is the clock signal
expressing the magnitude of the feedback voltage and proportions to
time differential of the phase variation component of the clock
signal 211. Digital data 216 output from AD converter 215 is fed to
personal computer to measure the magnitude of the component of
which frequency fr is 10 kHz.
[0103] FIG. 15 is the signal spectrum of each unit of the
communication system according to the first experimental example
shown in FIG. 14. The figure (a) shows the spectrum of signal light
204 before is filtered out by acousto-optic tunable filter 203
shown in FIG. 14. The figure (b) and (c) show spectrum changes of
signal light 208 passed through acousto-optic tunable filter 203.
These changes correspond to the sweep width .DELTA.f of sinusoidal
wave 172 (FIG. 12) expressed by curve 183 of FIG. 13. Deviations of
the optical frequency of the signal light 208 from the center
frequency of acousto-optic tunable filter 203 as the tunable
wavelength optical band pass filter are +10 GHz and -10 GHz,
respectively. Here, the frequency deviation of +10 GHz corresponds
to the change of the wavelength of -0.08 nm.
[0104] FIG. 16 shows the example the waveform of the phase
modulation component, which is detected by the PLL circuit, in
accordance with the time passed. This figure shows the example the
waveform of the phase modulation component of clock signal 211. In
this figure, the axis of abscissa represents the time passed and
the axis of ordinate shows the feedback voltage as PLL output
signal 214 in FIG. 14. On the other hand, the waveform in the top
end of the figure shows the case, where the dispersion value is -60
ps/nm and the waveform in the bottom end of the figure shows the
case, where the dispersion value is +60 ps/nm.
[0105] PLL circuit 213 itself has a wide bandwidth to receive
easily the noise. Therefore, a circuit noise and an optical signal
noise of PLL circuit 213 causes deterioration of an S/N ratio
(signal to noise ratio.) However, a sufficiently higher frequency
component than the modulated component of acousto-optic tunable
filter 203 does not influence the measurement accuracy and, hence,
communication system 200 of the first experimental example employs
a low pass filter, of which cutoff frequency is 50 kHz. By this,
the noise in a high frequency band is eliminated to improve the S/N
ratio. Though the waveform shown in FIG. 16 is expected to become
ideally a rectangular wave, the waveform is blunted because the low
pass filter cuts off simultaneously the high frequency band of the
signal. However, an absolute value of the dispersion value is
determined in the magnitude of the 10 kHz component and, thus,
deterioration of the measurement accuracy caused by this can be
neglected.
[0106] FIG. 17 shows the relation between the dispersion value of
the optical fiber and the feedback voltage as the detected phase
modulation component. The axis of abscissa represents GVD (group
velocity dispersion.) In other words, FIG. 17 shows the result of
measurement of the magnitude of 10 kHz component of the phase
modulation component of clock signal 211 for the dispersion value
of the optical fiber 205 (FIG. 14.) In communication system 200 of
the first experimental example in FIG. 14, the magnitude of the
dispersion value is measured by using the amplitude of PLL output
signal 214 output from PLL circuit 213. On the other hand, by a
phase relation between PLL output signal 214 and acousto-optic
tunable filter 203 as the optical band pass filter, in other words,
by delay or proceed of phase modulation component when the
wavelength selected by acousto-optic tunable filter 203 is moved to
a longer wavelength end, plus or minus of the sign is determined.
As described above, the magnitude of the dispersion and the sign
are separately determined. If the amplitude (the magnitude of the
phase modulation component of clock signal 211) of PLL output
signal 214 output from PLL circuit 213 is only measured, only the
absolute value of the dispersion is known. The sign expressing plus
or minus of the dispersion is important. Then, in FIG. 17, the
magnitude of the dispersion and the sign are separately determined
to express as the axis of ordinate. Meanwhile a position, where the
magnitude of the phase modulation component of clock signal 211 is
the minus, the sign is determined minus. From this figure, it can
be known that in the position, where the dispersion is zero, the
feedback voltage is not zero, offset occurs, and an accurate
dispersion can be determined from the magnitude of the phase
modulation component of the clock signal. The dispersion was
measured in a range from about -100 ps/nm to +90 ps/nm. This
limitation was caused by the range, in which the clock recovery
module as clock extraction module 174 can normally extract clock
signal 175.
SECOND EXPERIMENTAL EXAMPLE
[0107] From the first experimental example as described above, it
can be found that the dispersion of the optical fiber shown in FIG.
14 can be measured in high accuracy. As the second experimental
example, it is described that these dispersion values can be
measured in real time.
[0108] FIG. 18 shows the outline of the communication system used
in the second experimental example. The same reference numeral is
attached to the part identical with that of FIG. 14 and, thus,
description of the part is omitted properly. In communication
system 300 of the second experimental example as the embodiment of
the in-service optical pulse evaluation device of the third
example, optical fiber 305 is serially connected to tunable
dispersion compensator 306 between optical pulse transmitter (TX)
202 and acousto-optic tunable filter 203. Optical fiber 305 is a
single mode fiber having the dispersion value of 253 ps/nm. Tunable
dispersion compensator 306 used was a comparable with the tunable
dispersion compensator using temperature-controlled chirped fiber
grating (S. Matsumoto et al., IEEE Photon. Technol. Lett. Vol. 13,
no. 8, August 2001.) Behavior, when a specified dispersion value of
tunable dispersion compensator 306 was changed from -193 ps/nm
(total dispersion 60 ps/nm) to --313 ps/nm (total dispersion -60
ps/nm) and returned from -193 ps/nm to -313 ps/nm, was monitored by
the in-service optical pulse evaluation device. A measurement
interval was defined almost 150 msec.
[0109] FIG. 19 shows the example of measurement of the transient
response of the tunable dispersion compensator in the second
experimental example. The axis of abscissa represents the time
passed since the specified dispersion value of tunable dispersion
compensator 306 was changed from -313 ps/nm to -193 ps/nm. The axis
of abscissa expresses GVD (group velocity dispersion.) The
specified value was returned from -193 ps/nm to -313 ps/nm at 60
sec. The behavior of convergence on the specified dispersion value
for 10 seconds and, therefore, it was observed that high dispersion
can be accurately monitored.
[0110] In the second experimental example, when the measurement
time was shortened up to 10 msec being the limit of the performance
of AD converter 215, monitoring was possible. As described above,
communication system 300 of the second experimental example can be
used not only for a simple chirp measurement of the optical pulse,
but also as a real time dispersion monitor by using the
transmission signal.
THIRD EXPERIMENTAL EXAMPLE
[0111] As described above, it is known that the in-service optical
pulse evaluation device according to the present invention can
measure the dispersion of the optical fiber in the high velocity,
accurately, and in a wide range and, also, is suitable for the real
time dispersion monitor. In the third experimental example, a
simple optical network is practically made and the adaptive
dispersion compensation system is built up in combination of the
real time dispersion monitor with the tunable dispersion
compensator according to the present invention.
[0112] FIG. 20 shows the outline of the communication system used
in the third experimental example. Communication system 400 of the
third experimental example being an embodiment of the in-service
optical pulse evaluation device of the third example has
transmitter (Tx) 402 for sending RZ signal 404 of 40 Gbps. The
constitution of transmitter (Tx) 402 is same as that of optical
pulse transmitter (TX) 202 of FIG. 14. Signal light 404 sent from
transmitter (Tx) 402 is input in first optical cross connect node
(OXC) 406 as a light exchanger to select any one end of first path
407 or second path 408. First path 407 comprises the single mode
fiber having a length of 14 km, of which dispersion value is 223
ps/nm. Second path 408 comprises the single mode fiber having a
length of 17 km, of which dispersion value is 280 ps/nm. To the
other ends of first path 407 or second path 408 is selectively
connected an input end of second optical cross connect node 409 as
the light exchanger.
[0113] To an output end of second optical cross connect node 409 is
connected a receiver (Rx) 412 through tunable dispersion
compensator 411 using a temperature control type CFBG (Chirped
Fiber Bragg Grating) capable of adjusting the specified dispersion
value In receiver 412, real time dispersion monitor 413 is
incorporated. Receiver 412 supplies dispersion value 414, which is
output from this real time dispersion monitor 413, to tunable
dispersion compensator 411 and, by this, operates the feedback
control to make the dispersion zero. By using this communication
system 400 of 40 Gbps, in order to measure a dispersion tolerance,
the total dispersion value from transmitter 402 to receiver 412 is
changed to measure a bit error rate (BER:) by the receiving
end.
[0114] FIG. 21 shows the result of measurement of this bit error
rate. The axis of abscissa expresses the change of the total
dispersion value and the axis of ordinate expresses the bit error
rate. The dispersion tolerance was about .+-.30 ps/nm.
[0115] Next, during actual working of the adaptive dispersion
compensation system, paths 407 and 408 are switched by first
optical cross connect node 406 and second optical cross connect
node 409 to count error number for the period. The time necessary
for switching paths 407 and 408 was 10 or fewer msec. The
measurement time of the real time dispersion monitor 413 was 150
msec and the error number counting interval was 500 msec.
[0116] FIG. 22 shows the temporal change of the dispersion value
and the error count resulted from the adaptive dispersion
compensation experiment. The axis of ordinate expresses the time
passed and the top and the bottom of the axis of abscissa expresses
the dispersion value measured by real time dispersion monitor 413
and the counted error number, respectively. "Path 2" of this figure
means second path 408 and path 1" means first path 407. In the
point in which 5 sec passed, second path 408 is switched to first
path 407 and, in the point in which 25 sec passed, first path 407
is again switched to second path 408. In either operations, the
error free status was recovered within 1.5 sec. 1.35 sec excluding
150 msec being the measurement time of real time dispersion monitor
413 is the time required by the change to the specified dispersion
value of tunable dispersion compensator 411. From this figure, it
appears that the accuracy of the measured dispersion value is
somewhat bad. This is because the relation between the dispersion
value and the feedback voltage is subjected to linear approximation
in real time dispersion monitor 413 and also because the specified
dispersion value of tunable dispersion compensator 411 produced the
error.
* * * * *